Recently, as disclosed in Japanese Patent Publication No. 2005-344606, throttle valves are provided in branched intake air passages (intake manifolds) for respective cylinders of an engine, so as to respectively control the intake air amount. In addition, a communication chamber is provided to communicate the intake air passages with each other at respective downstream sides of the throttle valves. A nipple for negative pressure is provided at the communication chamber, and the intake negative pressure is applied from the nipple to a brake booster, so that the negative pressure for the braking operation is obtained.
In the above throttle system (the multiple-valve type throttle system), in which throttle valves are provided for the respective cylinders, intake air pressure largely varies at a downstream side of the throttle valve in each stroke (a suction stroke, a compression stroke, a combustion stroke, and an exhaust stroke), because a volume of the intake air passage is small at the downstream side of the throttle valve. A time period, during which intake negative pressure necessary for a braking operation can be obtained, is limited to such a period during which each of the engine cylinder is around BDC (bottom dead center) of the suction stroke. The time period for obtaining the intake negative pressure necessary for a braking operation is shorter than that for a general throttle system, in which a single throttle valve is provided at an upstream side of a surge tank to control the intake air amount. Therefore, it is disadvantageous in the multiple-valve type throttle system, in that it is not easy to obtain negative pressure required for a braking operation.
In particular, an engine is recently equipped with a PCV system (a positive crankcase ventilation system), an evaporated fuel gas treatment system (a fuel vapor treatment system) or the like, in order to reduce emission amount of HC into the air. According to such system, the crankcase ventilation gas or the evaporated fuel gas is introduced into the intake pipe at the downstream side of the throttle valve. Therefore, in the multiple-valve type throttle system having the small volume for the intake air passages at the downstream side of the throttle valve, the intake air pressure at the downstream side of the throttle valve is likely to increase by the introduction of the crankcase ventilation gas (the PCV gas) or the evaporated fuel gas, not only when the throttle valve is opened but also when the throttle valve is close to its fully closed position. As above, in the multiple-valve type throttle system, it is a problem that the negative pressure for the braking operation can not be easily obtained.